This is driving me nuts and it is to the point that I can't use my phone. I recently got an 8120 off ebay (unlocked, former AT&T).
Works fine with no Micro SD card. Used to be with the card in everything worked fine, just sometimes trying to scroll up on the trackball would hang.
Now, if there is a memory card installed, the unit will power cycle. The light will flash red and the phone will power down and restart over and over again.
It doesn't matter what size the card is. I started with an 8Gb, went to 4Gb, and now have a 2Gb. Doesn't matter.
I wiped/reset the phone and updated the software to the latest/greatest version I could find v4.5.0.182. Didn't help.
Has anyone out there had and resolved this issue? I really like my 8120 and want it back.
Thanks.

Your problem is not that unusual. Go to media area on phonen choose menu and format the card. If it doesn't change, its a circuitry issue with the phone. I am not sure that a piece of tape to tighten the card will help.Posted via BlackBerryForums.com Mobile